Many techniques, such as, for example, bandwidth compression, for increasing the information handling capabilities of communications systems exist. However, the desire for communications systems capable of transmitting even greater quantities of information in a given time period has almost inevitably led to the development of such systems capable of operating at ever higher frequencies. Communications systems using electromagnetic radiation were initially developed for operation at very low frequencies, less than 10 MHz, and the possibility of using electromagnetic radiation in the visible or near visible region has always been of interest because of the very high data rate transmission, relative to low frequency systems, possible in this short wavelength, high frequency region.
The lack of a suitable radiation source, which had long hindered developments in this area, was solved, at least in principle, with the invention of the laser, and the light source presently contemplated for most such systems is a semiconductor laser. Several transmission media are possible for use in communications systems operating in the visible or near visible region, but after the development of low loss glass transmission lines, commonly referred to as optical fibers, such optical fibers become the preferred transmission media. The optical fiber typically comprises a silica based glass having a high refractive index core surrounded by a low refractive index clad. The optical communications systems presently contemplated have a light source and photodetector optically coupled to each other by the optical fiber.
For transmission over extended distances, for example, more than 20 km, the optical signal is regenerated at one or more intermediate points by a device commonly referred to as a repeater. The repeater unit detects the incoming optical pulse and reshapes it into the desired electrical shape which is then applied to a laser. The repeater thus enables the system to operate over larger distances than are possible with a single fiber segment.
All optical fiber systems presently in commercial use are based on the encoding of the information by amplitude modulation (AM) and direct detection of the transmitted optical energy, i.e., they are two-level, one-channel systems. In other words, information is transmitted as an optical pulse is either transmitted or not transmitted within predetermined time intervals. However, more sophisticated schemes of encoding the transmitted information afford possibilities of either or both higher data transmission rates or longer repeater spacings than are possible with amplitude modulation. Multi-level and/or multi-channel systems should significantly increase the information transmission capacity of optical fiber communications systems. For example, optical frequency modulation (FM) might improve either the data transmission rate or permit the repeater spacings to be increased.
Although there has been interest recently in the modulation and demodulation of coherent laser radiation, the development of FM optical communications systems has been relatively slow. This is due to several factors including the absence of a laser that might be easily tuned through a suitable frequency range and the stringent requirements imposed upon the system by heterodyne detection. For example, the frequency shift keying (FSK) system described by Saito et al in IEEE Journal of Quantum Electronics, QE-17, pp. 935-941, June 1981, was a two-level, single-channel system using a continuously tuned laser and heterodyne detection. The frequency tuning rate is extremely small, approximately 100 MHz/mA, and limited to a tuning range of less than approximately 1 GHz.
However, the stringent requirements imposed on the system by heterodyne detection could be considerably relaxed if there were a laser easily tunable over a wide frequency width and which had a very narrow frequency output.